One technique of offshore well production includes a platform located above sea level. The platform has a surface wellhead assembly, and a string of conduit extends from a subsea wellhead assembly to the surface wellhead assembly. Production tubing for the flow of well fluid is suspended at the surface wellhead assembly and extends through the conduit into the well. The string of conduit may comprise an inner riser string that is lowered through an outer riser string extending between the subsea and surface wellhead assemblies. A seal seals between the conduit and the bore of the surface wellhead assembly.
During installation of the string of conduit, its lower end will first be tied back into the subsea wellhead assembly, then the upper end is hung off on a load shoulder in the surface wellhead housing. Preferably the conduit is supported in tension. The conduit is typically casing that may be approximately 30 to 40 feet in length, thus it is unlikely that an assembled string of conventional casing would be the correct length to extend between the wellhead assemblies at a desired level of tension. Upper casing joints could be changed out for ones of different lengths, but this method takes time. A variety of methods and devices are known for accomplishing this type of installation, but improvements are desired.